This application claims the priority of German applications 198 14 204.8 filed Mar. 31, 1998 in Germany, and 198 46 268.9 filed Oct. 8, 1998 in Germany, the disclosures of which are expressly incorporated by reference herein.
The present invention relates to an arrangement for condensing a drafted fiber strand in a condensing zone arranged downstream of a front roller pair of a drafting arrangement, which condensing zone comprises a stationary sliding surface having at least one suction slit extending essentially in a fiber transport direction and a perforated transport belt which transports the fiber strand over the sliding surface, to which transport belt a nipping roller, defining the delivery side of the condensing zone, is arranged.
An arrangement of this general type is described in U.S. Pat. No. 5,600,872. The drafting of the sliver or the roving ends at the front roller pair. A condensing zone is located between the front roller pair and a delivery roller pair arranged downstream thereof, in which condensing zone the drafted fiber strand is condensed transversely to the transport direction, so that when exiting the delivery roller pair, a spinning triangle does not arise when the fiber strand is subsequently twisted to a yarn. The perforation corresponds to the width to which the fiber strand is to be condensed. A belt cradle effects the guidance of the belt, which cradle takes the form of a suction device and comprises a sliding surface.
As a result of a delivery roller pair being arranged downstream of the suction device, the suction of the fiber strand ends at a clear distance from the nipping point of the delivery roller pair. The condensed fiber strand can, disadvantageously, spread out again before it reaches the nipping point. Thus the actual aim of the condensing zone is only partly achieved.
It is an object of the present invention to arrange the condensing zone downstream of the front roller pair of the drafting arrangement in such a way that the condensed fiber strand retains its condensed state until it reaches the nipping point.
This object has been achieved in accordance with the present invention in that the nipping roller presses the transport belt to a nipping point on the sliding surface and that the suction slit extends to the nipping point.
As the nipping roller does not operate in conjunction with another delivery roller, but rather with a stationary sliding surface, the suction can be applied up to the nipping point. The width of the condensing effect is not determined by the perforation of the transport belt, but rather by the width of the suction slit applied in the sliding surface, which suction slit extends up to the nipping point defined by the nipping roller. An advantage is attained in that the pneumatically generated condensing of the fiber strand is maintained also at the nipping point, so that subsequently a fiber strand, still in a condensed state, can be twisted into a yarn. The result is a yarn with a good material utilization and high tensile strength which is less hairy.
The condensing effect can be increased when the suction slit is designed tapering in transport direction of the fiber strand and/or when the suction slit extends diagonally to the direction of motion of the transport belt. In the latter case, the fiber strand obtains a slight false twist, by means of which, together with the friction effect of the transport belt, the outer fibers can be better wrapped around the fiber strand.
A similar effect can be achieved when the transport belt traverses transversely to its direction of motion.
The transport belt can have various designs. According to especially preferred advantageous embodiments, the transport belt is constituted as a narrow-mesh woven band, which surrounds a sliding surface in the form of a hollow profile. It is hereby favorable when the hollow profile, serving as a sliding surface for the transport belt, is curved in the fiber strand transport direction. In addition to the suction by the suction slit, this results in the fiber strand finding good support on the sliding surface.
In certain preferred embodiments, the hollow profile, in the form of a suction device, extends over a plurality of adjacent spinning stations, whereby as a result one suction device comprises a plurality of suction slits.
The speeds of the nipping roller and the transport belt should be only slightly higher than the speed of the front roller pair, namely by such an amount that a certain necessary tension is effected on the fiber strand. Further drafting is not necessary. In order to achieve this, the nipping roller is driven by a roller of the front roller pair of the drafting arrangement and drives for its part the transport belt. The nipping roller can be arranged together with the top roller of the front roller pair at a joint rocker, which is in turn arranged at the top weighting arm of the drafting arrangement.
The suction slit should be wider than the completed condensed fiber strand, for example 1.5 mm. The perforated area of the transport belt, in contrast, is significantly wider than the suction slit.